The present invention generally relates to detecting circuits and more particularly to an AM detecting circuit having improved linearity under small-signal conditions.
One type of detector commonly used in the prior art employs a series detector diode in which an intermediate signal is half-wave detected. As is well known, detector diodes have barrier voltages on the order of 0.2 to 0.6 volts. When an intermediate frequency signal having a voltage lower than this barrier voltage is applied to the diode, no response is produced and, when the barrier voltage is a significant fraction of the signal voltage, detection is performed but it is accompanied by distortion. To avoid distortion, such signal voltages should preferably be above 120 dB.mu. (=1V). In systems employing high circuit voltages, the relatively small barrier voltage is unimportant. However, in portable or battery-operated equipment wherein the highest supply voltage is only on the order of a few volts, it is difficult to provide a signal voltage with a high enough level to provide detection without distortion.
Other proposed detectors have employed an operational amplifier with a pair of oppositely poled diodes in parallel in a feedback path with a resistance in series with one of the diodes. Although this improved the level of signal detection, the lack of provision in such a circuit for applying a bias to the diodes still permits distortion to occur during small signal conditions.